1. Field of the Invention
The present invention relates generally to the field of overcurrent protection devices for poly-phase circuits and more particularly to a device that generates a control signal after the occurrence of a fault, provided that at the time of control-signal generation the current in at least one phase is below a predetermined load level and the current in all phases is below both a fixed blocking level and an adjustable trip level.
Various overcurrent protection devices generate trip signals in response to predetermined overcurrent conditions in poly-phase electrical circuits. One specific device of this type is the S&C Overcurrent Relay Type ZSD which is described in Photo Sheet 551.700 (Apr. 11, 1988) of S&C Electric Company, Chicago, Ill. This device is utilized to trip a switch operator to open an interrupter switch to achieve poly-phase isolation of the circuit after a fault has been cleared. This feature is accomplished by monitoring the current in each phase of the circuit and comparing the monitored current with a field-adjustable trip level setting in the range of 400 to 2400 amperes. If the current in any phase exceeds the level detector setting for a predetermined time period (to eliminate response to transient disturbances), the device starts a field-adjustable tripping circuit timer. If current above a specified level returns on all phases before the timer times out, the device is automatically reset. If such current does not return on all phases (as will be the case when a fault is cleared), when the timer times out, a trip signal will be generated to activate the switch operator-- thus opening the interrupter switch. A blocking circuit is employed which inhibits the generation of the trip signal in the event that the timer has timed out but the over-current has not yet been successfully cleared.
While this device is generally suitable for a variety of application, in other applications, the trip level setting may be set below the 600-ampere blocking level setting; e.g., the trip level setting may be in the range of 400-600 amperes. Thus,. there could be situations where a fault in the range of 400-600 amperes would still be present and the associated load-break switch would be asked to interrupt the fault current--a duty for which it is not designed. To alleviate this situation, the fixed blocking level could be lowered to 400 amperes or the minimum trip level setting could be raised to 600 amperes. However, this is not entirely satisfactory since those levels are different to satisfy legitimate load circuit concerns and to give a greater range of selective control.